1. Field of the Invention
The field of technology of the present invention relates generally to measurement systems, and more specifically to test units for testing components of chemical activity measurement devices.
2. Discussion of Related Art
There are a considerable number of processes involving the use of chemical solutions where it is essential that the solution have a given concentration of ions. By way of example, when corrosion resistent coatings are formed on aluminum with compounds of fluorine it is important to have a given concentration of active fluoride, i.e., fluoride ions. When a single aluminum structure is to be coated, the amount of active fluoride can be determined from the molar concentration of fluorine. However, when successive coatings of uniform color and appearance are to be formed, it is necessary to replenish the solution in such a way as to maintain a consistent level of fluoride ion activity regardless of the molar concentration of fluorine.
Fluoride ion concentration can be determined by finding the rate at which the solution etches soda lime glass. The glass is weighed, and after it is exposed to the solution for a given time interval at a carefully controlled temperature, the products formed by the etching process are washed away and the glass is weighed again. The loss in weight of the glass divided by the time interval gives the desired etching rate that is proportional to the concentration of fluoride ions. But, since the time required for an accurate measurement may take several days, this method is impractical for continuous production processes.
In order to obtain an instantaneous indication of the fluoride ion concentration, a monitoring device is immersed in the solution. It is comprised of a reference electrode, a lanthanium fluoride ion selective electrode and a preamplifier for amplifying the voltage between the electrodes. In order that the gain of the amplifier remain constant in the presence of temperature changes, a temperature compensation resistor is coupled to it. The output voltage of the amplifier is transmitted to a remote control station where it is used to control the addition of chemicals to the solution so as to maintain a desired fluoride concentration. Because the corrosive effect of the solution on the components, they have to be checked from time to time to ensure that they meet specifications required for proper operation.
A straight forward way to check the components is to physically substitute them for components in an on-line monitoring device that is known to operate properly, but experience has shown that the components are often damaged so that a good component is destroyed. Unfortunately, this is especially true of the expensive measuring electrode.
A similar situation is encountered with monitoring devices for the pH value of solution used in cleaning processes.
3. Summary of the Invention
In accordance with this invention, a portable test unit is provided having components like those of the monitoring device and a voltmeter for indicating the output of its preamplifier. The test unit permits components of the monitoring device to be tested remotely from the site where the monitoring device is used. Provision is made for electrically substituting the components of a monitoring device for those of the test device while the electrodes of both are exposed to the solution and their temperature compensation resistors are at the same temperature. Possible damage is avoided because the components do not have to be removed from the monitoring device. If the reading of the voltmeter does not change significantly, substituted components are considered to meet specifications.
In a preferred embodiment, the test unit is provided with a power supply that can be plugged into a service outlet and a shielded housing containing circuits for making necessary interconnections between cables for electrodes and a cable for the preamplifier, the power supply and the voltmeter. Also included therein is a thermal compensation resistor and a switch for coupling it or a thermal compensation resistor of a monitoring device to the preamplifier.
In accordance with another aspect of this invention, all of the components of a monitoring device can be substituted for the components of a test device. A cable is provided that carries operating potentials provided by the power supply to the preamplifier and conducts its output voltage to a terminal that is connected to the voltmeter. The cable is terminated in a plug that fits pins on the amplifier. By coupling this plug to the corresponding pins on the preamplifier of a monitoring device, that preamplifier receives operating potential, and its output voltage is supplied to the voltmeter of the test unit.
In order to reduce the likelihood of false operation, the application of voltages for testing a preamplifier and the substitution of the temperature compensating resistor of a monitoring device for that of the test unit are effected by spring-biased switches.
The test unit could be powered by batteries, but it is preferable that it have a power supply that is adapted to be plugged into the power service system of the plant in order to make it less likely that the test unit will suffer damage by being used in the corrosive atmosphere in the vicinity of a coating machine.